1. Field of the Invention
This invention relates to the perforation of oil and gas wells to achieve maximum production of oil and gas, and more particularly, to a self-contained, downhole drilling apparatus for drilling production drain holes at right angles to a central well bore.
Production of hydrocarbons from oil and gas wells involves drilling a well bore to the producing interval and sealing a casing in the bore by pumping cement into the annular space between the bore and casing. Because of the varying porosity of the production intervals and other factors, it is necessary to provide lateral perforations or drain holes in the production interval for collection of the hydrocarbon fluid in the well bore. Conventional techniques for achieving this objective are frequently difficult to implement and the results sometimes defy accurate prediction.
The most basic and general benefit of a drain hole or drain holes in the production interval or zone of an oil or gas well is that of increasing the amount of formation surface exposed to the well bore. In addition to this desirable formation exposure, it has been found that there is a significant relationship between horizontal drain hole depth and an increase in bottom hole pressure. Productivity of a completed and perforated well depends upon the number of drain hole perforations in the production interval, the diameter and depth of penetration of the perforations, and the distribution of the drain hole perforations. Production in many wells has decreased over the years to the point of marginal profitability due to collapse of the drain hole walls, which reduces the flow of hydrocarbon fluid from the producing interval or "pay", into the well.
2. Description of the Prior Art
Conventional techniques for achieving production of hydrocarbons from the producing intervals of oil and gas wells include "perforating" the zone of production surrounding the well bore by means of perforating guns which are lowered into the well bore and fire projectiles through the well casing and concrete sheath, and into the production zone to create drain holes which allow hydrocarbons to flow from the production strata into the well bore. Other methods include the use of explosive, shaped charges which "jet" an aperture into the formation to create the desired hydrocarbon drainage. However, most of these devices do not have sufficient force to penetrate deep into the formation, and many times, the projectiles or explosive charges must penetrate two strings of casing, two cement sheaths and a wall of hard rock. Improved techniques which use equipment capable of achieving deeper projectile penetration and maximum backsurging to clean the drain holes are still capable of effecting only limited penetration into the production strata, even under ideal conditions. Other techniques include using chemical charges such as thermite which are ignited and create an extremely hot charge which burns through the casing, concrete and hydrocarbon-bearing zone. However, these devices are sometimes unreliable and are often not able to penetrate far enough into the "pay" zone to achieve a significant increase in hydrocarbon flow.
Other devices designed for lowering in the well for the purpose of boring and drilling holes at right angles to the well bore at the production interval, have been proposed, but many problems have been encountered in these systems. For example, the relatively low bit rotation speed generally necessitated by using a curved shaft sometimes requires too much time to achieve significant penetration, and increasing the bit rotational speed and shaft load frequently causes failure of the shaft. Generally, conventional drilling devices have not proved capable of sustaining the high compressive loads necessary to penetrate the well casing, concrete sheath, rock and producing interval within an economical time frame without failure. Other problems have been encountered with bit retrieval and freedom of rotation of the drilling string.
An early device for drilling drain holes transverse to the well bore is disclosed in U.S. Pat. No. 1,367,042 to B. Granville. This apparatus includes a rigid pipe fitted with an elbow; a flexible tube positioned inside the pipe and extending through the elbow; a drill bit rotatably mounted at the outer end of the flexible tube; and a flexible shaft attached to the drill bit and lying within the flexible tube. A drive mechanism is attached to the flexible shaft for driving the shaft and rotating the bit.
Other methods and mechanisms for achieving transverse drilling in a well bore are taught in U.S. Pat. No. 3,958,649 to George H. Bull, et al, which mechanisms include a flexible, spring biased, telescopic fluid conduit fitted with a turbodrill on the lower end designed to extend from an opening in the side of an elongated, cylindrical housing. The conduit further includes multiple, axially aligned spools with interconnecting cylinders and compression rings in the cylinders to create the desired flexibility and telescopic action.
Another downhole drilling system is disclosed in U.S. Pat. No. 4,051,908, to W. B. Driver, which system includes a 90 degree elbow attached to the downhole end of a tube section having a small diameter. The top of the tube section is attached to a tubing string of relatively large diameter, which houses a downhole electric motor. A small drill bit and flexible drill pipe operate in the small tube section to effect drilling into the production formation.
Yet another perforating tool for drilling transverse holes in oil and gas wells as disclosed in U.S. Pat. No. 4,185,705 to Gerald Bullard. This tool includes a housing for insertion in the casing and provided with a detachable boot. A pair of motors are used to rotate a flexible drill and to advance and retract the flexible drill shaft in order to effect penetration of the production formation with the drill.
It is an object of this invention to provide a downhole, transverse drilling apparatus which is self-contained and can be lowered into a well bore to drill one or more drain holes of selected depth into a producing interval to increase the flow of hydrocarbons from the interval into the well bore.
Another object of the invention is to provide a downhole drilling apparatus for drilling drain holes at right angles to a vertical well hole or bore, which apparatus can be lowered into the well bore by wire-line, stabilized in the bore, and activated to drill holes of selected size and depth in the hydrocarbon-producing strata.
Yet another object of the invention is to provide a self-contained, downhole, transverse drilling apparatus for drilling drain holes in oil and gas wells by penetrating beyond the "well skin" into undamaged production formation and increasing the flow of hydrocarbons into the well bore.
A still further object of this invention is to provide a self-contained, downhole drilling device which can be lowered into a well bore adjacent to a producing strata or interval and activated to bore through the well casing, concrete sheath and production interval to create one or more drain holes for enhancing the flow of hydrocarbons into the well bore, and particularly, for enhancing hydrocarbon production from characteristically slow-flowing formations such as chalk, clay and tight sands and aiding in artifically induced secondary and tertiary hydrocarbon recovery systems such as water and fire floods.
Yet another object of the invention is to provide a transverse, downhole drilling system which is self-contained in a tubular housing and includes a drilling string or strings characterized by multiple tapered, splined segments which nest and interlock inside one or more telescoping guide tubes in such a manner as to permit sufficient lateral movement to traverse the 90 degree bend in the guide tubes, with a drill bit on the lower end of the string and a gear-engaging means on the top end for engagement with a drive system and effecting rotation of the string and drill bit.